In recent years, the use of insulated-gate bipolar transistors (IGBTs) or other semiconductor devices to control motors is increasing with a tendency towards lower pricing of the semiconductor devices, especially the IGBTs, in order to save energy. A circuit diagram of a motor drive for one arm of driving based on a conventional technique is shown in FIG. 2. In FIG. 2, the motor that is a load is replaced by an inductance element, which is connected between the high-potential side and output of a main power supply Vdd. The high-voltage terminal of the main power supply Vdd and the collector of an upper-arm IGBT are connected via wiring HL1. The emitter of the upper-arm IGBT and the output terminal of the main power supply Vdd are connected via wiring HL2. The collector of a lower-arm IGBT and the output terminal are connected via wiring L1. The grounding terminal of the main power supply Vdd and the emitter of the lower-arm IGBT are connected via wiring L2. A diode HDIODE is connected in parallel between the collector and emitter of the upper-arm IGBT. Also, a diode LDIODE is connected across the lower-arm IGBT. A load inductance element Lload is connected between the high-voltage terminal and output terminal of the main power supply Vdd. A driving circuit constructed of an HnMOS and an HpMOS is connected to the gate terminal of the upper-arm IGBT.
The upper-arm IGBT111 is connected to the output terminal, so in terms of potential, the upper-arm IGBT is driven in a floating state with respect to the grounding terminal of the main power supply. When the upper-arm IGBT is on, therefore, this IGBT is impressed with the same high voltage as that of the main power supply. Accordingly, the driving circuit 113 requires electrical insulation from the grounding potential of the main power supply. As described in Patent Document 1, the above conventional technique has used a photocoupler 130 to transmit signals to an insulated driving circuit 113. It is also described in Japanese Patent Laid-open No. 2004-304929, paragraphs (0002), (0020), (0021) that a level-shifting circuit is used as a means for sending a driving signal from a lower arm to an upper arm floating in terms of potential.
In FIG. 2, the source of a high-withstand-voltage MOSset for turn-on signal transmission is connected to a lower-arm grounding terminal. The gate of the MOSset is connected to a logical circuit. One end of a resistor Rset is connected to the drain of the MOSset. The other end of the resistor Rset is connected to the high-voltage side of a power supply HVcc for driving the upper arm. A Zener diode Zdset for protection from an overvoltage is connected across the resistor. The source of a MOSset, a high-withstand-voltage MOS for turn-off signal transmission, is connected to the lower-arm grounding terminal. The gate of the MOSreset is connected to the logical circuit. One end of a resistor Rreset is connected to the drain of the MOSreset. The other end of the resistor Rreset is connected to the high-voltage side of the power supply HVcc for driving the upper arm. A Zener diode ZDreset for protection from an overvoltage is connected across the resistor Rreset.
In synchronization with the rise of an upper-arm driving signal from a microcomputer or the like, the logical circuit uses the upper-arm driving signal to generate a turn-on signal in pulse form in the high-withstand-voltage MOSset for turn-on signal transmission. In synchronization with the fall of the upper-arm driving signal, the logical circuit also generates a turn-on signal in pulse form in the high-withstand-voltage MOSreset for turn-off signal transmission. The two MOS's are used to transmit signals to the upper arm more rapidly and with less power consumption. The resistor Rset is further connected to the setting side of a flip-flop (FF), and the resistor Rreset is further connected to the resetting side of the FF. The driving signal that was decomposed into the rising pulse and the falling pulse by the logical circuit is restored at the FF to the same pulse width as that of the original driving signal from the microcomputer. The output of the FF is reversed in state by a NOT circuit, and when the command from the microcomputer is “H” (high), the output of the FF becomes “H” and thus the output of the NOT circuit becomes “L” (low). This turns on the HpMOS, supplies an electric current from the upper-arm driving power supply HVcc, and turns on the HIGBT, the IGBT for the upper arm.